Gas turbine components are subjected to rigorous mechanical loading, thermal stress, oxidation, corrosion, and abrasion. Hot gas path components of such turbines are often made of nickel or cobalt based superalloys optimized for resistance to high temperature creep and thermal fatigue. Protective coatings are applied to increase durability and field performance at high temperatures. MCrAlY (where M represents a transition metal, and Y represents yttrium) is a material commonly used as a protective coating, especially as a bond coat for an overlying ceramic insulation as part of a thermal barrier coating (TBC) system. Such bond coats prevent the substrate from being deteriorated by oxygen, and they act as an intermediary to bridge the difference in the coefficients of thermal expansion (CTE) between the ceramic and metallic materials, thereby reducing stress levels.
MCrAlY materials have been optimized for thermal and chemical compatibility with the superalloys along with oxidation and corrosion resistance. In gas turbine components, the M in MCrAlY is normally nickel (Ni) and/or cobalt (Co). Nickel-based alloys provide superior oxidation resistance, and cobalt-based alloys provide superior corrosion resistance. The chromium (Cr) provides hot corrosion resistance, and aluminum (Al) aids in formation of a stable oxide barrier. Yttrium (Y) enhances adherence of the oxide layer. Elemental additions of cerium, silicon, lanthanum or hafnium to the bond coats are done to improve their performance in terms of the oxidation or thermo-mechanical behavior and ceramic coating adherence. Performance of a TBC often depends on the ability of the underlying MCrAlY bond coating to form a tenacious, protective, aluminum oxide scale that is thermodynamically stable, slow growing, adherent, and that inhibits interactions between the substrate surface and the outside corrosive environment.